I don't actually think it should be, but I can make a compelling case.
While it's possible to have a dictionary of all of the words of a language, it will always be missing the proper nouns. It will also be missing the data needed to take my very poor spelling and figure out what I'm trying to say.
Combining these two problems, I've found that oftentimes my local spell checker can't figure out what I'm trying to say, but Google can figure it out no problem.
I don't mind having to go to Google once in a while to do this manually, but that can is the case for a distributed spell check service.
Doesn't Google use a large corpus of documents along with machine learning/statistical analysis to do spelling and grammar checking? The same way they do language translation.
